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Rare Earth-Doped Phosphors for White LEDs Chapter 278 47 (Si,Al)(N,O)4 b a FIG. 45 Crystal structure of b-SiAlON. The unit of bond distance is nm. Picture was drawn based on the structure data obtained from the database (ICSD #8263). Al2O3, in which a Si–N pair is replaced by a pair with an equivalent charge, such as Al–O; it is then described by the general formula Si6zAlzOzN8z. b-SiAlON has a hexagonal crystal structure with space group P63 or P63/m (Jack and Wilson, 1972; Oyama and Kamigaito, 1971). This structure is shown in Fig. 45. The phosphor b-SiAlON is known to form a 3D network of corner-sharing (Si,Al)(O,N)4 tetrahedra with infinite channels parallel to the c-direction (Kimoto et al., 2009). Xie et al. (2007) investigated in detail the effect of the material composition (z value in Si6zAlzOzN8z) and Eu2+ content on its luminescence properties. The samples with lower z values showed a relatively high solubility of Eu2+ without structural change as well as high phase purity, fine particle size, and little particle aggregation. The high- est green emission intensity was obtained for Si Al O N :Eu2+ . The 5.5 0.5 0.5 7.5 0.03 excitation spectrum monitored at 536 nm consists of a broad absorption band extending from 250 to 530 nm (Fig. 46), in which two distinguishable peaks appear at around 303 and 400 nm accompanied with a shoulder at 480 nm, showing strong optical absorption from near-UV (400–420 nm) to blue (420–470 nm) lights available from LEDs. The emission spectrum under exci- tation at 450 nm exhibits a single broad band with a maximum at 536 nm. The FWHM value is approximately 55 nm, which is smaller than that of the conventional green-emitting Ba-rich (Ba,Sr)2SiO4:Eu2+ ($65 nm) and SrSi2O2N2:Eu2+ ($82 nm) phosphors (Kim et al., 2005a,b; Li et al., 2005; Mueller-Mach et al., 2005), indicating higher color purity of b-SiAlON:Eu2+. In addition, the phosphor shows low thermal quenching. With increasing tem- perature up to 150°C, the emission intensity decreases by only 12–14% without any emission band shift (Chung and Ryu, 2012; Xie et al., 2007). A white LED for a back-lighting unit combined with a blue LED, a commercial CASN (CaAlSiN3:Eu2+) phosphor, and b-SiAlON:Eu2+ shows a high-CRI white light (Chung and Ryu, 2012). The color gamut of the white LED is 102% of NationalPDF Image | HANDBOOK ON THE PHYSICS AND CHEMISTRY OF RARE EARTHS
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